Biological implants such as artificial bones or tooth roots have recently attracted attention because when bones or teeth are broken or otherwise lost by an accident, etc., the implants can be bonded to the remaining bone or implanted in the bones of the jaw and thus can be used in a form close to natural bones or teeth and ensure maintenance of comfortable daily lives. Since these implants are to be embedded in the body, they should essentially be required to be nontoxic to the body. They are also required to have various other properties, such as sufficient strength, moldability, freedom from dissolution, moderate specific gravity, and biocompatibility.
Metals have been used as biological implants such as artificial bones or tooth roots from the standpoint of their physical strength and workability. Previously, noble metals were used in consideration of effects on the body, but have gradually been replaced by alloys such as stainless steel as a result of development of alloys having good corrosion resistance. Metallic materials containing cobalt as a main component have also been developed and come into use as biological implants.
Among these metallic materials, noble metals are stable, but have the defect of high price, high specific gravity, and high weight. Alloys such as stainless steel have good corrosion resistance, but sometimes contain substances which will cause toxicity when dissolved in vivo. Hence, the alloys are not always versatile, and also have too high a weight as shown by their specific gravity of about 8.
Recently, titanium (d.sup.20 =4.50) or titanium alloys which are nontoxic, stable and light in weight with a relatively low specific gravity came into use.
These metallic materials have sufficient mechanical strength and good workability, but as such, they have the common defect of lacking affinity for bone tissues in vivo.
On the other hand, research has been done on the use of ceramic materials which are stabler and lighter than metals. As a typical material, alpha-alumina is known. This substance is chemically stable, nontoxic, and light and has very high mechanical strength. However, it has the defect that its workability is much inferior to metals, and as such, it lacks affinity with bone tissues. Stabilized zirconia has also come into use because of its good toughness, but has the same defect as alpha-alumina.
As a stable material, a glass material whose surface is mainly rendered porous is also known, but has the defect of insufficient mechanical strength, lack of affinity for the human body, and difficulties in processability.
Recently, apatite ceramics were proposed, which provide a solution to the problem of lack of biocompatibility, which has been a common defect of conventional materials. The main inorganic component of bones or teeth is a calcium phosphate compound (composed mainly of hydroxyapatite), and the apatite ceramics containing this compound as a main component have very good affinity for bones and very good adaptability after being embedded in the body. However, even apatite ceramics which seem to be ideal in some respects have the defects of low mechanical strength, poor moldability, and poor processability, and are limited in use.
order to overcome these defects, it has been desired to develop metals or ceramic materials having satisfactory biocompatibility by coating apatite on the surface of metals or ceramics. This requires a technique of bonding metal to ceramics or ceramics to ceramics, but the only method now known therefor is plasma spray.
Although the plasma spray method is useful in such bondings, it has the defects that the entire surface of a material having a complex shape is difficult to coat, the entire surface of a porous material cannot be coated, it requires an expensive device, the ratio of utilization of expensive apatite particles is low, and the adhesion between the coating and the substrate is not entirely sufficient.